A wavelength conversion device is used to obtain ultraviolet light from continuous visible laser radiation by wavelength conversion using a nonlinear optical material having optical anisotropy, such as a uniaxial crystal. This nonlinear optical effect is a series of phenomena based on nonlinear polarization induced in the nonlinear optical material. Using a second-order nonlinear optical effect from among them, wavelength conversion (the generation of a second harmonic, added frequency, differential frequency, and so on) can be done from laser light. For example, it is usual to convert a light wavelength through second harmonic generation (SHG) with a nonlinear optical material such as a beta barium borate crystal (BBO). Thus, the simplest example of such wavelength conversion is second harmonic generation, and the frequency of the converted light is twice as much as that of the incident light and, hence, the wavelength of the converted light becomes half. Simply describing wavelength conversion of the second harmonic generation as an example, the added frequency generation of two wavelengths and differential frequency generation of the two wavelengths are also similar to this example when incident light is composed of the two wavelength components. In addition, since most nonlinear optical materials are of crystalline form, they are referred to as a crystal hereafter.
Since the amplitude of induced nonlinear polarization is proportional to the square of the amplitude of the electric field of incident light in the secondary nonlinear optical effect, the converted light power is proportional to the square of the incident light power, but the proportional constant is fairly small. Thus, generally, the light power converted with small conversion efficiency is low.
As shown in FIG. 20(a), a wavelength conversion device 100 for the simplest second harmonic generation comprises a crystal 104 located at a radiation site of laser light in a laser device 102. Laser light radiated from the laser device 102 enters the crystal 104, from which light L2 (hereafter referred to as SHG light) at twice the frequency of the incident light is radiated in the same direction as laser light L1 (hereafter referred to as excitation light) entering the crystal 104.
Although the obtained power of the SHG light depends on the characteristics of the crystal, the diameter of a light beam, the excitation light power, etc., an output power becomes small if continuous light with a small power is the excitation light since the power of the SHG light (output power) is proportional to the square of the excitation light power, as mentioned above. Usually, since the power of the laser device 102 is about 1 W, conversion efficiency in this case is very low, less than one thousandth. Hence, an external resonator is used to increase the power efficiently to convert a wavelength of laser light with a low power. Thus, to increase the power of the SHG light, it is necessary to increase the excitation light power entering the crystal. For example, if the excitation light power can be increased by 30 times in the resonator, the power of the SHG light is increased by 900 times and, hence, if the conversion efficiency is 1/2000, approximately 45 percent of the excitation light can be converted to the SHG light.
A wavelength conversion device 120 shown in FIG. 20(b) comprises an external resonator (hereafter referred to as a resonator) 114 composed of a pair of mirrors located approximately in parallel. In this type of wavelength conversion device, the wavefront of an incident light is adjusted so that the phase front of incident light coincides with a reflection plane of the mirror at the light incidence section. In the resonator adaptively disposed in this way, since wavefronts of both rays of laser light coincide with each other, the excitation light is enclosed. In this case, if the power of the excitation light L1 entering the crystal 104 is increased by 100 times as much as the power of laser light L0 through making laser light L0 radiate from the laser device 102 go back and forth within the resonator, the power of the SHG light L2 increases by 10,000 times.
However, since excitation light L1 passes the crystal 104 in both forward and backward paths within the internal region formed with the pair of mirrors 112, the SHG light is generated in both forward and backward paths, and then the obtained SHG light becomes SHG light L2 and L2a radiated from the pair of mirrors 112. Thus, only half of the converted SHG light becomes available and the remainder is wasted.
The wavelength conversion device 120 shown in FIG. 20(c) comprises a resonator 124 composed of a set of mirrors 112 and 122 located approximately in parallel. In the wavelength conversion device 120, one mirror 122 of the resonator 124 is formed so as to reflect both the excitation light and SHG light in high reflectance, the other mirror 112 is formed so as to reflect only the excitation light and transmit the SHG light, and consequently SHG light L2 is obtained. Since the converted SHG light is transmitted through the crystal 104 again, however, interference occurs with the newly generated SHG light. Since the power of the outputted SHG light changes widely due to this interference, fine adjustment becomes necessary. In addition, the characteristic change of the crystal 104 and resonator 124 due to temperature change should be avoided as much as possible. Furthermore, the resonator should control the optical path length for both wavelengths of the excitation light and SHG light. Since these restrictions and controls are generally difficult to solve, it is very difficult to use them for a wavelength conversion device.
In a wavelength conversion device 130 shown in FIG. 20(d), a ring resonator 134 is used as a resonator so as to transmit the excitation light in the same direction through the crystal 104. Since this ring resonator 134 has at least three mirrors 112 formed to reflect only the excitation light and transmit the SHG light, optical paths are formed so that the optical paths reflected by each mirror 112 are arranged in a ring and, consequently, the excitation light goes back and forth within the resonator without interference; only the excitation light in the same direction is transmitted through the crystal 104, and the SHG light in the same direction is radiated.
However, using only the ring resonator, adjustment is complicated due to the increase in the number of optical elements, and its cost is expensive due to the increase in the number of elements.
The purpose of the present invention is to provide a device and method for waveform conversion capable of efficiently converting a light wavelength in a simple structure without complicated adjustment, taking into account the above facts.